New solutions for addressing the non-idealities present in semiconductor devices allows for continued reductions in scale and increases in performance. One of these non-idealities is interfacial traps, also referred to as surface states or interface states. Interfacial traps have wide-ranging and degrading effects upon the operational behavior of Metal-Oxide-Semiconductor (MOS) devices. Interfacial traps are allowed energy states in which electrons are localized in the vicinity of a material's surface. Interfacial traps primarily arise from unsatisfied chemical bonds that occur at the semiconductor-oxide interface. When a silicon lattice is abruptly terminated along a given plane to form a surface, one of the four surface-atom bonds is left dangling. The thermal formation of an SiO2 layer over the silicon substrate ties up some, but not all of the bonds present at the silicon surface. It is these unsatisfied chemical bonds, referred to as dangling bonds, that become interfacial traps.
Interfacial traps charge and discharge as a function of bias, thereby affecting the charge transport characteristics in a semiconductor device. As a result, interfacial traps produce instabilities in the operation of MOS devices through causing shifts in the threshold voltage and reduction in the channel conductance. When present in a significant concentration, interfacial traps are detected, for example, in the spread out the Capacitance-Voltage (C-V) curve measurement of an MOS device.
Hydrogen passivation is a common technique to minimize the presence of dangling bonds present at the SiO2—Si interface. Low-temperature, post-metal annealing in a hydrogen ambient is widely used in MOS fabrication to form SiH bonds with the silicon dangling bonds, thereby passivating them. However, these SiH bonds are not stable, and can be broken by highly energetic carriers (hot carriers) moving in the channel. The bond breaking process is referred to as de-passivation or desorption.
Hot carriers are energetic charge carriers that undergo ballistic transport across the channel between the source and drain. While traversing the channel between the source and drain, the population of hot carriers experience few, if any scattering events allowing for these carriers to achieve a velocity considerably higher than the normal distribution. The continued reduction in the scale of MOS devices leads to channel lengths of such short distance that the resulting increase in the population of hot carriers significantly magnifies the severity of the problem.
The deleterious effects of hot carriers is referred to as hot carrier degradation, (HCD). HCD is a particulary important reliability issue, given the desire to scale devices to extremely small dimensions. HCD results from heating and subsequent injection of carriers into the gate oxide or the Si substrate, causing a localized and nonuniform buildup of interface states and oxide charges near the drain junction of the MOS transistor. In addition, the generation of such interface and oxide defect states are in general accompanied by threshold shifts. The threshold voltage shifts lead to another phenomenon referred to as negative bias temperature instability (NBTI). Such reliability degradation phenomenom are frequently accompanied by mobility degradation, as exhibited by reduced transconductance and drain current. These effects are in general exponentially accerlated with increasing temperature.
The hydrogen-silicon bonds that are created by the hydrogen passivation processes to militate against defects formed at the Si—SiO2 interface, can be broken by hot carrier interactions, allowing for the release of the passivated defects that manifest themselves in a host of reliability and device performance problems. In addition, with desorption of the Si—H bond, the release of H can be involved in the further degradation of device reliability. For example, it is known that HCD and NBTI effects are always accompanied by the generation of positive charge into the gate oxide (SiO2), close to the Si—SiO2 interface. It is believed that this effect can be explained by the release of hydrogen and its incorportation, (positive charge), into the gate oxide layer as a result of the breaking of the Si—H bonds by the action of highly energetic carriers.
Deuterium (D) provides an improved way for stabilizing dangling silicon bonds. The Si-D bonding configuration has three distinguishable and highly favorable features when compared to the Si—H configuration with regard to bond stability and the relative equilibrium concentrations of the two species.    (1) The higher mass from the deuterium atom results in a lower diffusion coefficient than hydrogen. The lower diffusion constant of deuterium increases the rate of bond formation with silicon at the interface due to its longer residence time, i.e a factor in the rate of D bonding is determined by the rate at which D2 moves away from the surface. This is the case for the rate determining (non-equilibrium) part of the process. In simple terms, the D bonding rate is equal to the arrival rate of D atoms minus the outflow rate controlled by diffusion. A kinetic model might assume that the bonding rate is propotional to the number of dangling bonds available at any time t, i.e an exponential (decreasing) time dependence of the number of bonds created.    (2) The equilibrium concentration of Si-D, is found to be more than an order of magnitude higher than Si—H, and is related to both the ratio of the effective masses and their respective vibrational frequencies.    (3) It has been demonstrated both experimentally and theoretically that the Si-D bonds are significantly more stable than the Si—H bonds. The effect follows from the difference of the lattice dynamical behavior of isotopes and is referred to as the “giant isotope effect”. Current theories propose to explain this remarkable isotope effect, i.e., the Si-D bond is more resistant to hot-electron excitation than the Si—H bond as well as the order magnitude higher equilibrium concentration in Si, compared to that of hydrogen. The Si—H/D bond breaking at the SiO2/Si interface is caused by two competing processes. One is that the energy of the bonds is accumulated through excitation by energetic hot electrons. The other process is de-excitation, where the bond energy is taken away by coupling between the Si—H/D vibrational modes and substrate phonons. Van de Walle and Jackson suggested that the vibrational frequency of Si-D bending mode is close to the Si—Si TO phonon mode, resulting in energy coupling between the Si-D bending mode and the Si—Si TO phonon mode. This deexcitation effectively strengthens the Si-D bond. On the other hand, because the vibrational frequency of the Si—H bond is far away from the Si—Si TO phonon mode, there is no energy coupling between the Si—H bending mode and the Si—Si TO phonon mode, leading to Si—H bonds more vulnerable to hot-electron excitation.
Therefore, the larger equilibrium population of surface dangling bonds exhibited by deuterium as compared to hydrogen, in addition to their higher stability, provides significant improvements for the passivation process.
Finally, experimental evidence shows that the slight increase in bonding energy of the Si-D bonds is not the primary mechanism that provides higher stability over the Si—H bonds. It has been shown, for example, that the desorption of both Si—H and Si-D bonds can occur at carrier energies significantly lower than that predicted by a bonding threshold mechanism. Thus, the other factors described above, in particular the differences between the local vibration characteristics and their coupling to the silicon lattice phonons, dominate their desorption properties.
For the purpose of achieving the maximum concentration Si-D bonds for effective passivation, the presence of existing H commonly introduced by CMOS processing steps must also be considered. It has been shown that D is able to displace a considerable fraction of the existing H present for the case of optimal annealing temperatures. However, it has also been shown that re-exposure to D can still increase the concentration of D.
There is also a deleterious effect brought about by H or D passivation that must be considered. In the case where a MOSFET device receives a blanket passivation of H or D, i.e. the source drain areas in addition to the channel regions, it is known that both species can “passivate” the dopant atoms. In this case the dopant atoms become electrically neutral, increasing the series resistance of the source drain areas. This parasitic resistance effect degrades the device performance and manifests itself in lower drive currents with concomitant poorer performance.
There is another manifestation of the blanket method to introduce hydrogen (or deuterium) into the Si—SiO2 interface. The existing methods to introduce either hydrogen or deuterium into the channel interface towards the end of the device fabrication steps requires that either passivation species must diffuse through a number of film layers. In actual experience, some success has been achieved with the introduction of hydrogen by this method. However much less success has been experienced in the case for the blanket introduction of deuterium. One interpretation of these results from the experimental evidence is that the passivation by hydrogen is achieved from internal sources of hydrogen, e.g. poly-Si during the annealing processes. Then the failure of the blanket deuterium annealing processes can be explained by the fact that deuterium cannot diffuse through the multi film layers in the gate stack, as in the case for hydrogen, but in this case there is no internal source of deuterium. It is therefore highly desirable to develop new MOS manufacturing processes that efficiently incorporate the selective introduction of deuterium into the interface region that is stabilized within the channel of MOSFET devices.